About 70 percent of homes in the county are on septic systems, which leach nitrogen into the water table -- primarily from urine, said Kevin McAllister, head of the environmental group Peconic Baykeeper.

While current nitrogen levels are safe for drinking water, the amount that seeps into bays and tributaries produces readings well above what they should be for surface waters, McAllister said. That causes algal blooms, such as red and brown tides.

The Southampton plan, sponsored by Councilwoman Christine Scalera, is small. It would set aside $50,000 in an incentive program for replacing septic tanks installed before 1981. The program would offer a 50 percent to 60 percent replacement-cost incentive for replacing old septic systems with ones up to the current standard, which can cost $5,000 or more.

Christopher Gobler, a professor at Stony Brook University who studies algal blooms, applauded the effort as at least symbolic.

"It's a recognition by public officials that the septic problem is a concern," he said. "Even if it's a small program, it's a pioneering program for Suffolk County. It's pointing in the right direction."